Recently there has been considerable interest in reduction-oxidation (redox) active compounds for use in electronic displays. Such electrochromic materials are suitable for use whenever a light absorbing, rather than a light emitting, display can be employed. Electochromic materials are often preferred because of their low power consumption and their ability to operate without a constantly applied power source.
A number of compounds which change color as a result of electrochemical reactions are known. For example, polyvinylferrocene, polynitrostyrene or polymers from hydrolytically unstable ferrocene monomers have been confined to the surface of electrode materials. All of these materials are electrochromic. Polynitrostyrene, a pale yellow solid, is claimed to give an electrode a red appearance upon reduction whereas ferrocene-centered polymers turn from yellow/orange to blue upon oxidation. These results are encouraging, but the ferrocenes are too weakly colored and the reduction of polynitrostyrene is not readily reversible in the presence of moisture.
Compounds formed from 4,4'-bipyridinium, known as viologens, also exhibit electrochromic behavior. For example, dialkyl-4,4'-bipyridinium di-cations and their associated anions (dichloride, dibromide or di-iodide) form contrasting colors. These viologen di-cations range from colorless, yellow, and to red, while mono-cations are a striking blue-purple. Two basic problems with viologens are their long term stability as coatings and their sluggish switching speeds (rates of color change) at typical coating or solution concentrations.
There exists a need for electrochromic materials which can be immobilized, and preferably covalently bonded, upon electrode surfaces and which can be repeatedly subjected to redox actions in display systems. Moreover there is a need for electrochromic materials that can respond rapidly to an applied potential and yield a strong color (i.e. high molar absorbivity).